I agree, the power amp sim could have been better and this update has also reignited my interest.
The IR-D tone is based on a modded JTM45 circuit and so it's the slightly more vintage sounding of the two tube preamp pedals.
I daresay you could dial it in to get close to a 1959 sort of tone, based on Henning Pauly's demo where he puts it up against a Marshall SV20H and tries to EQ it to sound similar.